Recently we completed a preliminary report (Science, in press) which describes the first nonresonance Raman spectra of the in situ components of the photoreceptor membrane. Our data, reveal new structural information on opsin. In particular, we present preliminary evidence suggesting that the conformation of opsin is predominantly alpha-helical. This finding is in agreement with recent electron microscopic data on the opsin-like protein found in Halobacterium halobium membrane, but the biophysical probe of electron microscopy cannot work unless the membrane protein is in a highly ordered spatial arrangement, whereas this is not a requirement for vibrational spectroscopy. The Raman spectra of opsin membranes compare favorably in signal/noise ratio with spectra reported for simpler biological materials. Spectra recorded from opsin membranes prepared at different times are virtually identical. Spectra recorded at room temperature and 10 C also did not differ significantly. Most of the peaks found can be associated with protein and lipid group vibrations. Assignments were made, based on comparison with the Raman spectra of various proteins and phospholipids. We confirmed many of these assignments by measuring the Raman spectra of the separated lipid and protein components of the membrane. BIBLIOGRAPHIC REFERENCES: K.J. Rothschild, J. Andrew, W.J. deGrip, and H.E. Stanley "Opsin Structure Probed by Raman Spectroscopy of Photoreceptor Membranes", Science 191, 1176-1178 (March 1976).